The present invention relates generally to adjustable speed drives (ASDs) and, more particularly, to a system and method for stabilizing the output current of ASDs when using small DC link thin film capacitors.
In a conventional ASD, an alternating current (AC) power input is converted to a direct current (DC) power on a DC link by a rectifier and then to an AC power output by an inverter. The AC power output has the desired characteristics for operating an AC motor or other AC driven load. Often, a large electrolytic capacitor bank measuring between a few hundred and thousands of microfarads (μF) is used on the DC link to stabilize the DC link voltage and decouple the inverter side motor control of the ASD from the input rectifier operation. These electrolytic capacitor banks have a large capacity for energy storage and keep the DC link voltage fairly steady. Although electrolytic capacitors are effective to stabilize the DC link voltage, they have several drawbacks.
As one example, when a front end diode rectifier is used in the ASD, the AC input source current becomes severely distorted by the electrolytic capacitor bank such that low order harmonics pollute the utility grid. These harmonics can cause higher root mean square currents through connected transformers and feeder equipment. Sensitive equipment like instrumentation, computers, and communications systems may fail to function correctly or suffer damage.
As another example, electrolytic capacitors dry out and have a limited lifetime, which is a reliability concern. Then, upon replacement of an electrolytic capacitor bank, the capacitors need to be reformed or conditioned if they have been in storage for an extended period of time. This results in an inefficient installation process. The electrolytic capacitors will lose their charge in storage, so they need to be pre-charged. If the electrolytic capacitor banks are not pre-charged before energizing the ASD and the ASD does not have a pre-charge circuit, a high inrush current can flow through the rectifier and into the electrolytic DC link capacitor bank.
Because of the drawbacks of using electrolytic capacitors, smaller thin film capacitors are being used as a replacement. Thin film capacitors provide enhanced reliability, improved input current harmonic performance, reduced system size and cost, and out of the box installation with unlimited shelf life. However, thin film capacitors generally cannot provide the stability that electrolytic capacitors can. A significant amount of total harmonic distortion (THD) and DC link voltage ripple may thus be present in the system when thin film capacitors are employed.
In order to overcome the instability of the thin film capacitors, stability control strategies have been developed for ASDs. However, these control strategies generally include complicated algorithms that are not intuitive for users. In addition, while the control strategies may make the system more stable, they still do not provide enough stability for the ASD to produce a sine wave current, which is the ASD output for operating motors. Instead, the ASD output may approximate an oscillating wave, for example.
It would therefore be desirable to provide a system and method for analyzing and controlling the stability of an ASD with a small DC link thin film capacitor bank such that the ASD is stable under all normal operating conditions and the ASD output approximates a sine wave with low THD and DC link voltage ripple.